Even useful weeds are pests: Ethnobotany in the Bolivian Andes
Weed scientists, agronomists and an anthropologist in Bolivia surveyed farmers' practices and studied the ethnobotany of weeds. The hypothesis tested was that farmers managed weeds so as to take advantage of their uses. Farmers weeded row crops twice per cycle. Crop rotations usually began with potatoes and ended with an Old World cereal, broadcast in stands too dense to weed. Many weeds were fed to cattle, and fodder is the only use that requires more than an armload of weeds. Other uses of weeds (e.g., for home remedies) require just a few plants. Although most weeds have uses, they must still be controlled. The most important consideration regarding weeds is not their uses, but the fact that they are pests.
International Journal of Pest Management (2005) 51 (3) 189-207 [DOI:10.1080/09670870500213760]